Infrared photo-responsive elements have been found to be particularly useful for sensing the shape and features of remote objects for such purposes as multispectral mapping of the earth's surface. Such applications usually require an array of discrete photo-responsive elements to be positioned at the focal plane of an optical instrument. Current efforts towards producing higher resolution images has created a demand for a focal plane with a greater number of photo-responsive elements because the degree of resolution obtainable is proportional to the number of elements in the array. By way of illustration, one array under consideration has between one and ten thousand elements arranged in a linear array. The demand for higher resolution can be met only by such expedients as reducing the size of the photo-responsive elements because optical considerations tend to limit the area of focal planes.
To derive intelligible image data from the stream of signals generated by focal plane array at least two leads must be used to individually interconnect each photo-responsive element to its own preamplifier or other signal processing step. Typically, the elements in a focal plane array are made with pairs of discrete metal contacts. A pair of fine gold wires serves as interconnecting leads between each pair of contacts and a corresponding signal processing stage. The presence of the contacts and interconnecting leads places an upper limit upon the extent of size reduction achievable with available photo-responsive elements.
The sheer number of interconnecting leads itself presents a serious fabrication and reliability problem which may be exacerbated if the focal plane area must also accommodate cryogenic cooling equipment for the photo-responsive elements. Although available monolithic extrinsic silicon infrared detectors and charge coupled devices eliminate the necessity for interconnecting wires, such devices are not suitable for remote sensing of objects illuminated by reflected sunlight because available extrinsic silicon is not responsive to infrared radiation between 1.65 .mu.m and 2.3 .mu.m, and, therefore, is responsive to only part of the spectrum within one of the atmospheric windows.